French authorities have vowed maximum security for the European Championships that start in June but top officials and experts admit the country is defenceless to head off an Islamic State attack.

Hundreds of thousands of supporters of the 24 nations taking part are expected in France for the tournament that starts June 10. But militant attacks in Paris in November and Brussels this week have already cast a shadow over the event.

Euro 2016, which is guaranteed to draw the global spotlight with players such as Cristiano Ronaldo, Wayne Rooney and Zlatan Ibrahimovic, is impossible to protect, French experts acknowledged.

Euro 2016.

"We are really freaking out over the Euro," a member of the French anti-terrorism services told AFP on condition of anonymity.

"I took part in a meeting with people from the organising committee. They were considering a possible attack, wondering whether matches could start the next day or the day after.

"I was fuming. I got up and said: 'How many deaths does it take to stop everything'."

The worst fears surround fan-zones, with giant screens and food and alcohol, in the 10 host cities that could have between 10,000 and 100,000 people in them.

Interior Minister Bernard Cazeneuve said this week that France must not give in to terrorism and cancel any event. But opposition conservative deputy and security specialist Eric Ciotti said the government had to look at the fan zones and decide "on a case-by-case" basis whether to keep the zones.

The anti-terrorism official called the zones a "nonsense".

Fan zone risk

"How can you protect them. We know that several cities are seriously worried and are considering cancelling them."

The official said there are not enough private security guards in France to police the zones. "And anyway they cannot stop much."

The November 13 attacks on Paris in which 130 people died, with gunmen setting off bombs outside a stadium or spraying cafe terraces with machine gun fire, showed there are too many "soft targets" with vulnerable civilians.

The 81,000 capacity Stade de France just outside Paris, which was targeted on November 13, will host the opening match and final of Euro 2016.

One French police source acknowledged to AFP that it was "impossible" to stop attacks.

"But you can't stop such an event even if there is a high terrorist threat and an attack is possible," said the source.

"Holding matches behind closed doors or calling off fan-zones would not be the solution and gives a terrible image of the event."

Yves Trotignon, a former analyst with the DGSE external intelligence service, said the worst "nightmare" is that a big Brussels-style team of attackers is preparing for Euro 2016.

"They have all the time to prepare. They know the dates, the venues. They have time to check everything. It's like attacking a bank. These people can spend months studying the operation," said Trotignon.

The second threat is a lone attacker, acting alone and almost impossible to catch in advance because there are no telephone calls or emails, he added.

For France, Euro 2016 is important as it could influence the choice of the 2024 Olympics where Paris is a contender against Budapest, Los Angeles and Rome.

"The security is important for France," said Pascale Boniface, director of the Institute of International and Strategic Relations (IRIS) in Paris.

"But the truth is there is no zero risk. There is nothing easier than organising an attack. You protect 1,000 targets and it is the 1,001st which is targeted.

"Driving past a group of pedestrians to open fire could be done by a lot of people," said Boniface.

The French government sought again Thursday to insist that everything is being done to protect the 24 teams and their followers.

"Security will be a priority of the organisation of Euro 2016," said Secretary of State for Sport, Thierry Braillard.